JPS643888B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improved particulate polytetrafluoroethylene random copolymers containing small amounts of at least one selected fluoroalkylethylene in copolymerized form and to methods for making such copolymers. Granular polytetrafluoroethylene molding resins are used in the manufacture of billets and other plastic products by molding and ram extrusion. Granular polytetrafluoroethylene is produced by a method in which tetrafluoroethylene is polymerized with little or no dispersant and vigorous stirring to form a precipitated resin. This resin is commonly referred to as a "granular" resin, and the process is called "suspension polymerization." When the word "granular" is used in this case, it is used to define a resin made by suspension polymerization. Another type of polytetrafluoroethylene material is what those skilled in the art refer to as "fine powder" polytetrafluoroethylene. To make this so-called "starch", a method called "aqueous dispersion polymerization" is used. This method uses sufficient dispersant and gentle agitation to create a dispersion of fine colloidal sized particles in the aqueous reaction medium. This aqueous dispersion polymerization prevents resin particles from settling (ie, agglomerating) during the polymerization reaction. The dispersion can be used as is, or
In the separation process, dispersed particles may be agglomerated to obtain "fine powder." These two polymerization methods produce different products. Granular products can be molded into various shapes, whereas fine powders produced by aqueous dispersion cannot be molded and are coated with dispersion.
coating) or paste extrusion with the addition of a lubricant. In contrast, granular resins cannot be paste extruded. Tetrafluoroethylene polymers produced by aqueous dispersion polymerization are generally very flexible and easily deformable, and have a lower molecular weight than granular polymers, so the mechanical strength of the extruded resin is low. Not suitable for processing. Suspension polymerization of tetrafluoroethylene to obtain granular products generally involves using a polymerization reactor equipped with a stirrer system with a free radical-forming catalyst and optionally a buffer and optionally a small amount of a fluorinated emulsifier. It is carried out by filling it with an aqueous medium containing. Air is removed and tetrafluoroethylene is fed into the reaction vessel. After the start of the polymerization reaction, gaseous tetrafluoroethylene is supplied at a rate commensurate with the amount of polymer formed while keeping the polymerization pressure constant. It has long been known that the presence of small amounts of copolymerized monomers alters the properties of the resulting modified particulate polytetrafluoroethylene particles. For example, sinterability is improved and crystallinity is reduced. Two comonomers suitable for this technique are hexafluoropropylene, _CF3 -CF= _CF2 and perfluorinated propyl vinyl ether _{CF3CF2CF2 - O} -CF
= CF ₂ . This is clear from UK Patent No. 1,116,210 and US Pat. No. 3,855,191.
However, when perfluorinated propyl vinyl ether is used, the melt viscosity of the resulting particulate copolymer is relatively low, and large billets formed from it are
tends to deform (deflect) under its own weight during sintering. It would be desirable to discover copolymers that can be used to make particulate copolymers with higher melt viscosities. The use of hexafluoropropylene is also not entirely satisfactory. The presence of this comonomer reduces the thermal stability of the resulting copolymer compared to that of a tetrafluoroethylene homopolymer.
Evidence of such instability is found in U.S. Patent No. 3,855,391.
No. 1, column 1, lines 29-30. It is desirable to find comonomers that do not have a significant detrimental effect on the thermal stability of the copolymer. The present invention provides at least one formula Rr-CH=CH ₂ in which Rf is a fluorine-substituted alkyl group having 2 to 10 carbon atoms, and is perfluorinated or hydrogen or fluorine. A polymerized unit of a copolymerizable monomer of which is perfluorinated except for those in which one substituent selected from halogens other than halogen is bonded to the carbon atom of the Rf group. , to provide a granular modified polytetrafluoroethylene polymer comprising a tetrafluoroethylene polymer in an amount such that the melt viscosity of the resulting polymer does not become less than about 1×10 ⁸ PaS at 380°C. be. More particularly, according to the invention, at least
99.5% by weight of the repeating unit represented by the formula -CF ₂ CF ₂ - and 0.01 to 0.5% by weight of the formula -CH ₂ CH (Rf) - where Rf is 2 to 10 fluorine-substituted carbons An alkyl group having an atom, which is perfluorinated or perfluorinated, except that one substituent selected from hydrogen or a halogen other than fluorine is present bonded to a carbon atom of the Rf group. A copolymer comprising a repeating unit ^represented by A modified particulate polytetrafluoroethylene random copolymer is provided which is characterized in that it has a molecular weight sufficiently high to render it impregnable. The polymers of this invention are of high molecular weight as indicated by their inability to be processed by melt flow methods such as melt extrusion or injection molding. Therefore, these are “non-melt processable” (non-melt processable).
-melt-fabricable), i.e. it is not possible to extrude the melt. These are made by the tetrafluoroethylene suspension polymerization method and are therefore so-called granular tetrafluoroethylene polymers. In view of their inability to melt process, these copolymers generally have ^a
It has a melt viscosity of at least 100%. Such high melt viscosities are obtained by keeping the amount of comonomer low. That is, the amount of polymerized comonomer units incidentally present in the tetrafluoroethylene units is
It is 0.01-0.5% by weight of the total polymer weight. The comonomer has the following structural formula. R _f −CH=CH ₂ R _f is perfluorinated F(CF ₂ ) _o − (n is 2 to
It is an integer indicating the number of carbon atoms in 10. ), and may also be H or other halogens (Cl,
Br or I), preferably Cl or Br,
It may be perfluorinated except for one substituent replacing fluorine. Examples of comonomers that can be used to make the polymers of this invention include the following: CF ₃ −CF ₂ −CH=CH ₂ ; CF ₃
(CF ₂ ) ₉ CH=CH ₂ ; CF ₂ H−CF ₂ −CH=CH ₂ ;
CF ₂ H (CF ₂ ) ₉ −CH=CH ₂ ; CF ₂ Br−CF ₂ −CH=
CH ₂ ; CF ₂ Br (CF ₂ ) ₉ CH=CH ₂ ; CF ₃ −CFH−
_CF2 -CH= _CH2 , and similar substances. Mixtures of comonomers may also be used. Perfluorinated butylethylene _CF3- ( _CF2 ) ₃ -CH= _CH2 is preferred. The method used to produce the polymer of the present invention is the usual suspension polymerization method used to produce granular polytetrafluoroethylene. Such polymerizations are described in numerous patents, such as US Pat. No. 3,245,972 and US Pat. No. 3,855,191. Briefly, a conventional free-radical polymerization initiator is optionally mixed with a buffer and optionally in a _small amount (approximately
Tetrafluoroethylene is forced into an autoclave containing water along with an emulsifier (up to 200 ppm). The reaction mixture is stirred at appropriate temperature and pressure to effect polymerization. After the polymerization is complete, the polymer is isolated and dried. The comonomer may be precharged, but since the comonomer reacts much faster than the tetrafluoroethylene monomer, the comonomer units may be precharged throughout the polymer chain. In order to obtain a relatively uniformly distributed polymer in small amounts, it is preferred to add the comonomer over a period of 40 to 100% of the total polymerization time. Therefore, the copolymerizable monomer is preferably pre-charged into the polymerization tank, and then added during the polymerization, or
Or add by continuous addition. If too much comonomer is present, it will react by itself, so it is important not to add too much comonomer at once. The polymerization temperature is usually 50-120°C and the initiator is a peroxide or persulfate. Inorganic persulfates such as ammonium persulfate are preferred. Buffers that maintain the pH between 6 and 9 may be used if desired.
In addition, small amounts (e.g. relative to the water present)
(up to 100 ppm or 200 ppm) may be used to increase the surface area of the polymer particles producing dispersants. Stir thoroughly to ensure agglomeration of the polymer particles as the polymerization reaction progresses. The polymers according to the invention have a high melt viscosity and are therefore suitable for the production of billets.
(The billets are useful for tearing into tapes or sheets.) The polymers also exhibit high dielectric strength, making them useful in electrical grade tear tapes. It is useful for gaskets because the deformation under load is small. Bellows because of their high bending strength
is useful for membranes. It also has low gas permeability, making it useful in chemical linings and diaphragms. A distinction must be made in properties between the granular (suspended) polymers of the present invention and tetrafluoroethylene copolymers with higher comonomer content. The latter contain sufficient comonomer to enable the melt to be processed by extrusion methods customary for thermoplastics. The granular suspension polymer of the present invention, like unmodified polytetrafluoroethylene itself, belongs to the class of tetrafluoroethylene polymers that cannot be processed by melt extrusion, and can only be processed using special molding methods. There must be. Furthermore, since the polymer of the present invention is produced by suspension polymerization, it has a morphology different from that of a fine powder polymer obtained by dispersion polymerization. The latter polymers are produced in the presence of large amounts of emulsifiers and remain colloidally dispersed in the aqueous medium even after the polymerization reaction is complete. These finely divided polymers are obtained from a colloidal dispersion by agglomeration, and have an average particle size of 0.1 to 0.5 μm by aggregation.
agglomerates of colloidal primary particles of
is formed. As is known to those skilled in the art, this polymer can be processed by ram extrusion or mold sintering techniques, even if it contains copolymerized modifiers.
Cannot be molded. The properties of the polymers obtained in the Examples described later were measured by the following methods. (1) Quantification of copolymerization monomer content The copolymerization monomer content in the copolymer was quantified by Fourier transform (FT) infrared spectroscopy. A 10 mm cold rolled film was prepared and spectra were obtained using a Nicolet 7000FT infrared spectrometer at a resolution of 4 cm ^-1 . 880cm ^-1 −CH ₂
-Using refraction vibration (bending), correction was made using NMR analysis values. The absorbance of 880cm ^-1 is 888cm
It was calculated by taking the difference in absorbance between ^-1 and 880 cm ^-1 .
Perfluorinated butyl ethylene (PFBE) comonomer was calculated using the following formula. PFBE weight % = A880cm ^-1 - (0.000064 x t) x 100/t x 0.97 where t = thickness (mils), A = absorbance. (2) Standard specific gravity (SSG) The standard specific gravity of molding powder is ASTM D1457-69.
Measure the amount of water removed from a standard molded test piece according to the following. To make a standard molded part, 12.0 g of molding powder is 34.48 and
Preforming at a pressure of 6.90 MPa (352 and 70.4 Kg/cm ² ) followed by heating from 300°C to 380°C at a rate of 2°C per minute in a sintering process of the preform,
Hold at 380℃ for 30 minutes, then increase to 295℃ at a rate of 1℃ per minute.
Cool to temperature and hold at this temperature for 25°. Thereafter, the test piece is cooled to room temperature and its specific gravity is measured. (3) Specific surface area (SSA) SSA was measured with a “Quantasorb” surface area analyzer. This measurement was carried out using the crude raw material polymer obtained immediately after washing and drying from the reaction tank. (4) Melt viscosity Melt viscosity is calculated by measuring the tensile creep of a sintered piece held at 380°C. The details are as follows. 12 g of molding powder is placed between a 0.152 cm rubber retina and a paper spacer in a 7.6 cm diameter mold. Then,
Pressure is gradually applied to the mold until a value of 40.5 Kg/cm ² is obtained. After maintaining this pressure for 5 minutes, the pressure is gradually reduced. The sample disk is removed from the mold, the retina and paper spacer are peeled off, and then sintered at 380° C. for 30 minutes. Next, the heating furnace is cooled down to 290°C at a rate of about 1°C per minute, and the sample is taken out. Cut a crack-free rectangular strip of the dimensions shown below. Width 0.152 to 0.165cm, thickness 0.152
0.165cm to at least 6cm long. Measure the dimensions accurately and calculate the cross-sectional area. Both ends of the test strip are wrapped around a quartz rod using silver coated copper wire.
The interval between the wrapped parts is 4.0 cm. This quartz rod-sample assembly was placed in a cylindrical heating furnace with a length of 4
A cm specimen is heated to a temperature of 380±2°C. Then, attach a weight to the quartz rod at the lower end so that the total weight of the sample strip is about 17 g.
Measurements of elongation versus time are obtained and the best average value of the creep curve is determined at intervals of 100 to 130 minutes. Then, the apparent melt viscosity is calculated from the following relationship. ηapp = (WL _tg ) / 3 (dL _t /dt) A _T where ηapp = (apparent) shear melt viscosity, unit poise W = tensile load of the sample, g L _t = length of the sample (at 380°C ), cm (4.32
cm) g = gravitational constant, 980 cm/sec ² (dL _t /d _t ) = elongation rate of the sample under load = slope of the elongation versus time curve, cm/
sec A _T = cross-sectional area of the sample (at 380°C), cm ² The cross-sectional area increases by 37% at 380°C of the value at room temperature, etc. (5) Insulating strength Insulating strength is determined by molding 140g of powder into a mold with a diameter of 5.6cm, and heating at 6°C per minute from 290°C to 380°C.
It was heated at 380°C for 3 1/2 hours, and then cooled from 380°C to 290°C at a rate of 1°C per minute. Measurements were made on 5-ml tape using a Beckman Model PA5 AC Dielectric Strength Tester. (6) TGA TGA was measured using a DuPont 951TGA analyzer. Weight loss is the 2 hour value at constant temperature and 4
Time values were measured. (A negative weight loss actually indicates a weight gain.) (7) Tensile Strength and Elongation Tensile strength and elongation are ASTM-D-1457
-69. Examples Example 1 21.8Kg in 38 polykettle containers
demineralized water, 0.30g
(13.8 ppm) of ammonium perfluorinated octanonate (APFC) dispersant, and 3.0 g of ammonium persulfate (APS) initiator. The contents of the vessel were heated to 65°C and degassed. The vessel was stirred at 600 RPM using a two-bladed, 45° tilted stirrer. 1 ml perfluorinated butyl ethylene (PFBE)
was injected into the reaction vessel using a syringe. Then,
Tetrafluoroethylene (TFE) is placed in the reaction vessel.
It was added until the pressure reached 1.72×10 ⁶ Pa. Once the polymerization initiator is confirmed by pressure drop, add TFE to maintain the pressure at 1.72×10 ⁶ Pa and add an additional 8 ml.
PFBE was added continuously using a microfeeder at a rate of 0.1 ml per minute, ie over a period of 80 minutes. After adding the desired amount of TFE, the feed was stopped and the mixture was allowed to react until the pressure dropped to 5.5×10 ⁴ Pa. The total polymerization time was 95 minutes. Therefore, the ratio of the continuous feeding time of PFBE to the total polymerization time was 84%. 8.2Kg TFE
was polymerized. After degassing and creating a vacuum, the pressure in the reaction vessel was returned to atmospheric pressure using nitrogen, and the contents were cooled to below 50°C. The polymer was removed from the reaction vessel and the sticky material was separated. Polymer has an average particle size
Cut to 0.03mm or less. Various properties of the polymer are shown in Table 1. Examples 2-5 Example 1 was repeated using the following ingredient amounts.
The physical properties of the polymer are shown in Table 1.

【table】

【table】

Table Comparative Example A The properties of the copolymer described in Example 1 were compared with a copolymer prepared from tetrafluoroethylene and perfluorinated propyl vinyl ether (PPVE).
This comparative copolymer was prepared according to the method used to prepare the copolymer described in Example 1. Ingredients and other related manufacturing method data were as follows. APFC, g 0.3 APS, g 0.65 PPVE, pre-feed amount, M1 0.5 PPVE, continuous feed amount, M1 10 Microfeeder speed, ml/min 0.1 TFE, reaction amount, Kg 7.7 Reaction time, min 140 This PPVE modified tetrafluoro The physical property data of the ethylene copolymer are compared with that of the copolymer of Example 1 as follows.

[Table] Based on this data, PPVE modified polymer (comparative example)
However, even though SSG is substantially equivalent,
It can be seen that the melt viscosity is lower than that of the PFBE modified resin of Example 1. High melt viscosity is desirable for molding resins so that the heavy billet retains its shape at sintering temperatures. Comparative example B: 0.70g of APS, 0.30g of C-8, 13g of hexafluoropropylene (HFP) pre-injection,
A copolymer was prepared by reacting tetrafluoroethylene with HFP according to the method of Example 1 except that the temperature was 60°C. The amount ratio of copolymerized HFP in the copolymer was 0.07% by weight. The thermal stability of the HFP-modified copolymer was compared with the copolymer of Example 2, and the results were as follows.

[Table] A low TII means good stability. Therefore, it can be seen that the PFBE modified copolymers according to the present invention are thermally more stable. Comparative Example C A disc-shaped molded product (160 g) with a diameter of 5.6 cm of the polymer described in Example 1 and Comparative Example A (same as PPVE,
However, the polymer was prepared in a production scale amount by changing the formulation and only pre-dosing PPVE (mv=1.6×
Samples from 10 ⁸ PaS) were sintered in air at 380° C. for 5 hours with both molded parts covered with a weight of 4.8 kg on a disk. The sintered molded product had the following dimensions. Example 1 Comparative Example A Similar Sample Average thickness, cm 3.1 2.7 Top diameter, cm 5.5 5.8 Bottom diameter, cm 5.6 5.7 The molded product of Example 1 had a normal appearance, but the molded product of Comparative Example was considerably distorted. It was. The thickness of the comparative example (2.7 cm), which should normally increase during sintering, was smaller than before sintering (2.85 cm). Although the diameter (5.7-5.8 cm) should normally shrink during sintering, it was generally larger than before sintering (5.7 cm).

Claims (1)

[Claims] 1 At least 99.5% by weight of repeating units of the formula -CF ₂ CF ₂ - and 0.01 to 0.5% by weight of the formula -CH ₂ CH (Rf) - in which Rf is fluorine substituted an alkyl group having 2 to 10 carbon atoms, which is perfluorinated or has one substituent selected from hydrogen or a halogen other than fluorine attached to a carbon atom of the Rf group. A copolymer comprising a repeating unit represented by, which is perfluorinated except for the presence of fluorine, having a melt viscosity of at least 1×10 ⁸ PaS at 380°C. A modified particulate polytetrafluoroethylene random copolymer characterized in that the copolymer has a sufficiently high molecular weight to render the copolymer non-melt processable. 2. The copolymer according to claim 1, wherein Rf is perfluorinated. 3. The copolymer according to claim 2, wherein Rf is a perfluorinated butyl group. 4 At least 99.5% by weight of the repeating unit of the formula -CF ₂ CF ₂ - and 0.01 to 0.5% by weight of the formula -CH ₂ CH (Rf) - where Rf is 2 to 10 fluorine-substituted units an alkyl group having carbon atoms of A perfluorinated copolymer comprising a ^repeating unit represented by: In preparing a modified particulate polytetrafluoroethylene random copolymer having a molecular weight sufficiently high to render it non-melt processable, (1) tetrafluoroethylene and the formula Rf-CH=CH, _where Rf is A method characterized by carrying out suspension polymerization with at least one comonomer represented by the following, which is the same as above, and (2) adding the above comonomer for 40 to 100% of the total polymerization time.